This concerns a method and apparatus for signaling in a communication system.
A specific use for our invention is in private line data networks in which several subscriber terminals are connected over one or more private lines to a central processing unit (CPU). To provide for efficient use of available communication equipment one or more private line networks may be used to group together subscriber terminals in one locality and another private line may be used to link them to a centrally located computer. Advantageously, the subscriber terminals may be grouped together by a polled network which is similar to a party-line network in the public telephone system. Thus, the whole system may comprise a multitude of subscriber terminals, one or more polled private line networks, a second private line network, one or more remote terminals to link the polled networks to the second private line network, and the CPU.
Extensive description of typical communication systems and the apparatus used in them may be found in Bell Telephone Laboratories, Transmission Systems for Communications (revised 4th ed. 1971); in James Martin, Telecommunications and the Computer, (Prentice Hall, 1969); in U.S. Pat. No. 3,769,454 to R. A. Liberman, W. C. Bond, and E. J. Soltysiak, entitled "Method and Apparatus for Testing Teletypewriter Terminals", assigned to General DataComm Industries, Inc.; and in the Bell System Data Communications Technical Reference entitled "Characteristics of Teletypewriter Exchange Service", (September, 1970) available from: Engineering Director -- Data Communications, American Telephone and Telegraph Company, 195 Broadway, New York, New York 10007.
Of particular interest in any communication system is the interface between the communication line and the terminal. A modem or data set is ordinarily used to interface business equipment in a subscriber terminal to the communication line and another modem is used to interface that communication line at the remote terminal.
In any digital communication system, data are represented by different discrete levels of a parameter such as amplitude, frequency, phase or base band pulses. Most digital communication presently is binary in form, that is, each digital signal has one of only two possible states, a MARK, which is "1", or a SPACE which is "0", and represents a bit of information. As is well known, such data signals are transmitted at varying bit rates depending on the equipment used.
In numerous situations it is also necessary to transmit signals between the CPU and the subscriber which do not represent data. For example, when the CPU has data to be transmitted to a particular subscriber in a polled network it must also be able to activate that subscriber's terminal without activating all the other terminals connected to the network.
Several techniques are available for such non-data signaling in a data communication system. By far the most prevalent is the method of transmitting both data and non-data signals as patterns of MARKS and SPACES in one continuous bit stream in the same communication channel. By using a predetermined pattern of MARKS and SPACES as a sync signal to identify the beginning of a message and pre-specified formats for the location of subsequent non-data signals and data signals in the bit stream of MARKS and SPACES that is present on the communication channel, it is possible to separate the non-data signals from the data signals. Thus an individual terminal in a polled network can be activated by transmitting an address code and an operation code in the form of a data character which is recognized and acted upon by the terminal addressed in the address code. Alternatively, special codes such as pseudorandom patterns of MARKS and SPACES are used to distinguish control signals from data. Another possibility is to use a second communication channel for the control signals that must be sent between the CPU and the subscriber's terminal.
These techniques, however, have their limitations. Since the non-data signals in the first two techniques are patterns of MARKS and SPACES arranged in data character format just like the data characters in the data signals and since the way in which they are processed depends critically on the initial recognition of certain predetermined patterns of MARKS and SPACES in the bit stream of MARKS and SPACES, there is always the possibility that the pattern in a data signal might be the same as that in a sync signal or other non-data signal. This could lead to misinterpretation of a data signal as a non-data signal. With respect to use of a second communication channel for control signaling, such an alternative is generally unattractive economically.